1. Field of the Invention
This invention relates to a method for making ultra-fine ceramic particles by the application of plasma.
2. Description of the Prior Art
Ultra-fine ceramic particles with a particle diameter of less than 1,000 angstrom can be sintered easily at relatively low temperatures due to a large contribution of the surface energy thereof. The catalytic activities of the particles are also enhanced at relatively low temperatures for the same reason. Accordingly, the mass-production of ultra-fine ceramic particles at a low cost is widely desired.
The vacuum deposition (metallizing) method, for example, is a well-known procedure for making ultra-fine ceramic particles. This technique makes use of a slow deposition of the particles on a base at reduced pressures. This technique, however, is not suitable for mass production because it utilizes reduced pressures to effectively evaporate raw materials and consumes a large quantity of thermal energy, and because the formation rate of the particles is slow and the production rate of particles is low.
Another technique with an improved formation rate of ultra-fine ceramic particles utilizes a plasma jet. Metal powder is injected into a plasma jet to vaporize the metal in the technique. The metal vapor is then mixed with a reactive gas such as nitrogen in order to produce ultra-fine ceramic particles. Plasma Chemistry and its Application (Japanese title: Purazume Kagaku to Sono Oyo) published by Shoka-bo in Japan on Sept. 25, 1971 discloses a technique for making ultra-fine ceramic particles of TiN and Mg.sub.3 N.sub.2. In this technique, 200-mesh titanium or magnesium powder is injected into nitrogen plasma by a nitrogen carrier gas at a flow rate of 5 L/min, an electrical input of 12 KW, and a powder injection rate of 0.5 g/min. The extraction rates thereof were 30% for TiN and 40% for Mg.sub.3 N.sub.2 according to the book. However, it is considered to be difficult to employ this technique on a profitable basis. Although this technique of utilizing plasma as its heat source is more productive than the vacuum deposition method, this technique still produces a relatively small quantity of ultra-fine ceramic particles, because a relatively small quantity of metal powder is injected from the point of view that all the metal powder is vaporized.